There are a few bulbs....tulips & iris...in there that came up but no blooms. The grass is really, really thick.

My goal is to get rid of the grass, add some good soil and plant some flowers. I've tried to pull the grass after a rain - that lasted about 5 minutes until my arthritic hands began swelling. Help!!! anyone have any advise to a newbie gardener?

I would like to keep the bulbs that are in there (if possible). If I remove the bulbs and cover the grass with black plastic will it kill the grass? If so, how long does this take?

Then, I personally take card board. (go to your local appliance store and ask for all their large boxes.)
Put them down on top of the area (first maybe pull some weeds, the tall stuff and/or stuff with runners, if you can. Water first, easier to pull)

Throw down a bag or two or get a delivery from your local nursery some compost. Layer that down first. That gives your worms so stuff to eat. Then after you've laid the cardboard, wet it down, pretty good/so it softens up.

Aagin, compost on top of the cardboard. The compost will entice the card board to stay flat and the worms to have something to more to eat..... Then put a layer of chopped up leaves (in fall time/I personally feel that's the best route of items to use. Or should I say, that's what I would use to do my own beds.)
If no leaves are available, put very fine mulch. Not the chips, shredded. Cover it reasonably well, 3 or 4". It will render down.

I again, personally would let that sit....

If you're really antsy to garden, maybe the route to go is pots and planter boxes on top of all that. And a couple of cute statues and such, like a bird bath/butterfly bath, a couple pieces of stumps (to give heighth and to put stuff on top to cascade down to soften the stump. Nothing to poke through the cardboard. A few knick~knack's of something you collect. I collect frogs, so that's what I would do.
And leave it on till next spring........
It takes time to kill those buggers, any crack in the card board they seem to find. The crawlers and or runners. But because you've filled it with such good stuff for the ground, time comes, when you want to plant, the worms will have done most of your work for you by "tilling" from underneath. And if you still want to plant in the dirt, the weeds that might be tenatous enough to survive, woould then be so weak from being under the cardboard and weight of the compost, that they would just more than likely to rip right out........

My only other thought would be what kind of tree is it???????? Some trees have a very shallow rooting system.
How long has that existing bed been there? Any damage to the tree?
Some of the tree roots are imfamous for going into the good dirt and inflitrating it, from the bottom up.

I am going to plant a couple more trees - haven't decided which ones yet. I have a tulip poplar in front, back, and west side of the house. Besides the tulip poplar & hybrid poplar the front yard has a red delicious apple in the west front corner.

The yard is very, very large - I'm in the 'boonies' A "wet weather" creek is in front of my house and a year round creek comes from the north right to the west front (my house faces north) and then flows west along the front of the yard.
(Okay, now you know that not only am I a non-gardener, but not good with descriptions....LOL)

This spring I planted 6 red tip bushes along the bank.

I also have two pecan trees (never get pecans, but the squirrls do), two apple trees, a pear tree, and 2 peach trees. This spring we planted 4 grape vines and an apricot tree.

We have 4 raised vegetable beds, abt. 5' x 5'.